DC shunt-wound motors having shunt windings, which are supplied with constant current, are used for the main drive of rotary printing machines. The maximum continuous output of a DC shunt-wound motor is determined by a maximum temperature value. Provided no operational malfunctions or disruptions occur, this temperature is not exceeded even under continuous operation of the DC shunt-wound motor at its rated torque. When operational malfunctions occur, e.g. due to inadequate cooling of the motor or to extraordinary ambient conditions, for example, unusually high external temperature, overloading of the DC shunt-wound motor may occur. This can lead to damage or destruction of the motor.
As is generally known, thermocouples are installed in the stator windings of motors as a protective measure, and produce a signal by means of an evaluation unit at a given winding temperature, the signal being applicable for switching off the motor or for the implementation of other safety measures. A disadvantage of this solution for overheating protection are that these thermocouples are only in contact with the windings which are to be monitored, i.e. winding temperature is measured only indirectly. There is also the problem of insulating from potential-loaded i.e. voltage carrying components in the motor and in the thermo-sensor. A further disadvantage is the additional requirement for wiring connections in the terminal box, which can, inter alia, be mistakenly interchanged during connection of the motor. Furthermore, other supply lines are necessary, representing a further source of possible malfunction which may lead to failure of the drive. Extra amplifiers are required for evaluation of thermocouples and, under certain conditions, they must perform a linearization of the characteristic curve of the measuring element, and are to be calibrated, if necessary.
If non-continuous temperature sensors are used, several elements are necessary for monitoring and switching off.
A further disadvantage encountered with all thermocouples is that heating is measured at only one point, which means that the maximum temperature may, under certain circumstances, not be determined.
The monitoring of coolant flow in motors is also subject to a great susceptibility to malfunction and inaccuracy. Furthermore, only indirect evaluation of the actual motor temperature can be obtained by means of such coolant monitoring.
A further conventional facility for monitoring is provided by simulating the temperature behavior of the motor with the aid of temperature models. In this regard, the anticipated temperature over time is determined by means of measuring the operating current. Disadvantages of this system are that changes in ambient conditions or coolant flow cannot be detected and, furthermore, the preliminary condition of motor heating must be set, and also continued even with the drive shut off, respectively.